Robert S. Piascik

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andKmaxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H2O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O2, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3 and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.

Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- δK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for δK values where the cyclic plastic zone is sufficient to envelop subgrains. At low δK, when this crack tip process zone is smaller than the subgrain size, environmental FCP progresses on or near 100 or 110 planes, based on etch-pit shape. For inert environments (vacuum and He) and pure O2 with crack surface oxidation, FCP produces large facets along 111 oriented slip bands. This mode does not change with δK, and T1 decorated subgrain boundaries do not affect an expectedda/dN- δK transition for the inert environments. Rather, the complex dependence ofda/dN on δK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and 100/110 crystallographic cracking modes, the influence of cyclic plastic zone volume (δK), and the benignancy of O2. The SGC may be due to hydrogen production and trapping at T1 bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking.

Analytical Methodology for Predicting Widespread Fatigue Damage Onset in Fuselage Structure

A comprehensive analytical methodology has been developed for predicting the onset of widespread fatigue damage (WFD) in fuselage structure. The determination of the number of e ights and operational hours of aircraft service life that are related to the onset of WFD includes analyses for crack initiation, fatigue crack growth, and residual strength. Therefore, the computational capability required to predict analytically the onset of WFD must be able to represent a wide range of crack sizes, from the material (microscale) level to the global (structural-scale ) level. The results of carefully conducted teardown examinations of aircraft components indicate that fatigue crack behavior can be represented conveniently by the following three analysis scales: 1 ) small three-dimensional cracks at the microscale level, 2 ) through-the-thickness two-dimensional cracks at the local structural level, and 3 ) long cracks at the global structural level. The computational requirements for each of these three analysis scales are described in this paper.

Fatigue Crack Propagation in Aerospace Aluminum Alloys

This paper reviews fracture mechanics based, damage tolerant characterizations and predictions of fatigue crack growth in aerospace aluminum alloys. The results of laboratory experimentation and modeling are summarized in the areas of: (1) fatigue crack closure, (2) the wide range crack growth rate response of conventional aluminum alloys, (3) the fatigue behavior of advanced monolithic aluminum alloys and metal matrix composites, (4) the short crack problem, (5) environmental fatigue, and (6) variable amplitude loading. Remaining uncertainties and necessary research are identified. This work provides a foundation for the development of fatigue resistant alloys and composites, next generation life prediction codes for new structural designs and extreme environments, and to counter the problem of aging components.

An Overview of the Space Shuttle Columbia Accident from Recovery Through Reconstruction

The space shuttle Columbia launched from the Kennedy Space Center (KSC) in January of 2003. During ascent, between one and three pieces of material—likely insulating foam from the external tanks—impacted the leading edge of the left side of the orbiter. Upon re-entry back to earth, the Columbia began to disintegrate, leaving an enormous primary debris field stretching over eastern Texas and western Louisiana. Tens of thousands of volunteers were mobilized to help with the recovery of the Columbia remnants. Once the debris was delivered to KSC, several hundred scientists, engineers, and technicians helped analyze the debris and identify its original location on the orbiter. A Materials and Processes Team performed extensive failure analysis and chemical identification to help determine the most likely breach location resultant from the strike that occurred during liftoff, and the path that the impinging plasma generated during re-entry followed once it penetrated the wing of the Columbia. A combination of qualitative and quantitative analytical methods, ranging from radiographic nondestructive examination (NDE) and X-ray diffraction to scanning electron microscope with energy-dispersive spectroscopy (SEM/EDS) and electron probe microanalysis (EPMA), were used to help determine the breach location and the plasma path within the wing itself.

Textile Damage in Astronaut Gloves

The ability of protective gloves to resist cutting, tearing, puncture, abrasion, and unraveling is critical for many activities, but particularly for spacewalk activities. Since astronaut safety requires that pressure boundaries not be violated, damage observed in the outer layers of ten gloves after excursions about the International Space Station in 2006 was of great concern. An urgent effort was initiated to determine how and why the damage occurred and how to prevent it in the future. A team of scientists examined the failed fabric, yarns, and fibers of the damaged gloves with high-resolution microscopy and conducted laboratory experiments to produce glove damage under known load conditions. This article describes the damage observations and results of the laboratory tests, deduces how the damage occurred, and presents guidelines for designing gloves that are more damage resistant.

Micromechanics Modeling of Fracture in Nanocrystalline Metals

Abstract Nanocrystalline metals have very high theoretical strength, but suffer from a lack of ductility and toughness. Therefore, it is critical to understand the mechanisms of deformation and fracture of these materials before their full potential can be achieved. Because classical fracture mechanics is based on the comparison of computed fracture parameters, such as stress intlmsity factors, to their empirically determined critical values, it does not adequately describe the fundamental physics of fracture required to predict the behavior of nanocrystalline metals. Thus, micromechanics-based techniques must be considered to quanti@ the physical processes of deformation and fracture within nanocrystalline metals. This paper discusses hndamental physics- based modeling strategies that may be useful for the prediction Iof deformation, crack formation and crack growth within nanocrystalline metals. Introduction Fracture processes in materials such